1. Field of the Invention
The present invention relates to a method of preparing a carbon substrate for a gas diffusion layer of a polymer electrolyte type fuel cell, a carbon substrate prepared by using the method, and a system for manufacturing the same, and more particularly, to a method of preparing a carbon substrate for a gas diffusion layer of PEFCs by using carbon precursor staple fibers in an oxidized form and binder polymer staple fibers to skip an oxidation treatment process, thereby solving problems of deviation and low processability of a carbon substrate product and continuously forming a carbon substrate through a multi-step rolling curing process by using two or more press roll units to enhance the workability and maintain a uniform thickness of the carbon substrate; a carbon substrate thereby prepared; and a system for manufacturing the same.
The present invention is supported by the program of the Ministry of Knowledge Economy (Project No.: 2008NFC12J0125002008, leading institution: Hyupjin I&C, Co., Ltd., Research & Project Name: “Development of high performance, low cost GDL localization technology for hydrogen fuel cell automobile”).
2. Description of the Related Art
Fuel cells are electrochemical devices which generate electrical energy through electrochemical reaction of fuel and oxygen, and may be classified into a polymer electrolyte membrane (PEM) type, a phosphoric acid type, a molten carbonate type, a solid oxide type, and an alkaline aqueous solution type according to the types of electrolyte used in the cells.
Among the various types of fuel cells, polymer electrolyte fuel cells (PEFCs) are characterized by lower operational temperatures, higher efficiency, higher current and power densities, shorter start-up time, and a more rapid response to changes in load than other types of fuel cells.
PEFCs may be classified into two types: direct methanol fuel cells which utilize methanol as a fuel and hydrogen fuel cells which utilize hydrogen as a fuel. PEFCs have a structure in which a plurality of membrane electrode assemblies (MEAs) are stacked. MEAs include a fuel electrode (anode) and an air electrode (cathode) facing each other; and an electrolyte membrane interposed between the two electrodes, wherein each of the fuel electrode and the air electrode includes a gas diffusion layer (GDL) to which catalyst particles applied. GDL is formed by coating a microporous layer (MPL) on a carbon substrate made of a porous carbon membrane.
As shown in FIG. 1, a method of preparing a carbon substrate constituting a gas diffusion layer according to a conventional technology includes a carbon precursor fiber preweb formation process S10 for forming a preweb (a carbon precursor fiber preweb) composed of carbon precursor staple fibers, a first carbonization process S17 for carbonizing the carbon precursor staple fibers of the preweb by heating the preweb in an inert atmosphere at high temperatures into carbon staple fibers, thereby forming a carbon fiber web, an impregnation process S20 for impregnating the carbon fiber web with a thermosetting resin and carbonaceous fillers, a curing process S30 for applying heat and pressure to the carbon fiber web in which the thermosetting resin and the carbonaceous fillers have been impregnated to cure the thermosetting resin, and a second carbonization process S40 for carbonizing the thermosetting resin and increasing the crystallinity of the carbon staple fibers by heating the cured carbon fiber web in an inert atmosphere at high temperatures, thereby obtaining the carbon substrate.
FIG. 2 illustrates in detail the method of preparing a carbon substrate constituting a gas diffusion layer according to a conventional technology shown in FIG. 1.
As shown in FIG. 2, in addition to the carbon precursor fiber preweb forming process S10, the first carbonization process S17, the impregnation process S20, the curing process S30, and the second carbonization process S40, the method of preparing a carbon substrate constituting a gas diffusion layer according to a conventional technology further includes an oxidization process S16 prior to the first carbonization process S17 for preliminarily oxidizing the carbon precursor staple fibers. The carbon precursor fiber web forming process S10 may include an opening step S11 for opening the carbon precursor staple fibers, a carding step S12 for carding the opened carbon precursor staple fibers to form a carded thin carbon precursor fiber preweb, a crosslapping step S13 for crosslapping the carded carbon precursor fiber prewebs to form a thick carbon precursor fiber preweb, a binding step S14 for binding the crosslapped thick carbon precursor fiber preweb to obtain a bound carbon precursor fiber preweb, and a winding step S15 for winding the bound carbon precursor fiber preweb, while the second carbonization process S40 may include a carbonization step S42 and a graphitization step S44.
Referring again to FIG. 2, an opening machine is used in the opening step S11 to loosen the lumps of carbon precursor staple fibers, such as poly acrylonitrile (PAN)-based staple fibers, pitch-based staple fibers, and the like and disentangle them. In the carding step S12, a carding machine is used to arrange the carbon precursor staple fibers in parallel and then collect them to form a thin carbon precursor fiber preweb in a sheet shape. In the crosslapping step S13, the thin carbon precursor fiber preweb discharged from the Carding MC is stacked into a plurality of layers to obtain a thick carbon precursor fiber preweb having a desired weight. Carbon precursor staple fibers are mechanically entangled with each other from the thus-crosslapped thick carbon precursor fiber preweb by needle-punching using a special needle in the binding step S14 to obtain a bound carbon precursor fiber preweb. The carbon precursor fiber preweb which has been subjected to the needle-punching is wound in the winding step S15. As described above, the thus-wound carbon precursor fiber preweb is subjected to an oxidation process S16 in an air atmosphere at about 200° C. to about 500° C. and then to a first carbonization process S17 to obtain a carbon fiber web. Subsequently, the web is subjected to an impregnation process S20, a curing process S30, and a second carbonization process S40 composed of a carbonization step S42 and a graphitization step S 44 to obtain a carbon substrate.
As described above, according to a conventional technology, since carbon precursor staple fibers in the unoxidized form, such as PAN-based staple fibers, pitch-based staple fibers, and the like are used, it is possible to use a general non-woven fabric production process as it is, a carbon substrate may be prepared only by carbon precursor staple fibers without a binder polymer and it is possible to control the surface properties of the carbon staple fibers. However, the conventional technology is disadvantageous in that the carbon substrate obtained may have deviations in thickness and weight by ±20%, mainly due to the use of the general non-woven fabric production process, and in that the carbon precursor fiber preweb may be easily elongated during the production process due to properties of carbon precursor staple fibers which are ductile and thus a deviation in its properties and thicknesses may easily occur.
In addition, when the conventional technology is used, additional process costs are generated because an oxidation process of a carbon precursor fiber preweb takes about 1 hour. Furthermore, the shrinkage rate of a carbon precursor fiber preweb and/or web may non-uniformly occur even at low elongation, and thus the preweb may be of different thicknesses at different sites. In particular, a deviation is significant in the cross-machine direction. In addition, because there is no binder or binder fiber in the oxidized carbon precursor fiber preweb, the preweb may be easily elongated non-uniformly in the machine direction (MD) and the cross-machine (CD). Thus, it is difficult to control resin impregnation and web thickness uniformly in the impregnation process. In addition, although the strength of carbon precursor staple fibers may be further increased by elongation and other mechanical properties thereof are enhanced, the staple fibers may not be elongated to have a sufficient strength because a polymer web is oxidized.
In the curing process S30 of the conventional technology, when a carbon precursor fiber web including a thermosetting resin and carbonaceous fillers is put between plates of a hot press heated to a curing temperature at a predetermined spacing and heat and pressure is applied thereto, the thickness of the carbon precursor fiber web is decreased while the thermosetting resin is cured. During the curing process S30, thermosetting polymer resins impregnated between carbon precursor staple fibers and carbon precursor staple fibers, between carbon precursor staple fibers and carbonaceous fillers, and between carbonaceous fillers and carbonaceous fillers are cured by heat and pressure and their molecular weights are increased to have a binding force.
In the curing process S30 usually used, a carbon precursor fiber web is put between two plates which have been heated to a curing temperature or more and pressed for a predetermined time to cure the thermosetting resin by using a hot press process. However, because a roll-shaped product may not be continuously prepared by this method, a technology disclosed in US laid-open patent No. 2008-0258206 allows a predetermined length of a carbon precursor fiber web to be supplied at a predetermined rate. Then, a hot press is used to press the carbon precursor fiber web for a predetermined time and transfer the fiber web again. Through a stamping operation of the repeated pressing and transferring, the carbon precursor fiber web is pressed into a predetermined thickness.
However, in the conventional method of preparing a carbon substrate, as described above, the curing process is conducted in such a manner that only a predetermined length of the carbon precursor fiber web is heat-pressed for a predetermined time. Thus, it is problematic in that the thickness of the carbon substrate is not easily controlled because the curing process is not continuously performed and the curing is not uniformly achieved. In addition, because the curing process is performed by a flat-plate hot pressing process, it is disadvantageous in that the curing of the binder polymer resin proceeds under a rigorous constraint of the flat-plate and thus the flexural strength of the resulting carbon substrate is very low. When the flexural strength is low, it is disadvantageous in that it is difficult to perform roll-winding and the carbon substrate may be easily broken even by a relatively weak force.